1. Field of the Invention
The present invention relates to an opto-magnetic recording medium used for an opto-magnetic memory and the like and, more specifically, to an opto-magnetic recording medium capable of thermo-magnetically recording data by radiation of light beams and reading out recorded data by using a magneto-optical effect.
2. Related Background Art
The conventional opto-magnetic recording medium has a structure as shown in FIG. 1. A substrate 1 is formed of glass, plastic, or the like. A magnetic film 2 serving as a recording layer and an SiO.sub.2 protective layer 3 are formed on the substrate 1. As the magnetic film 2, a rare-earth iron amorphous alloy thin film is often used because of easy fabrication. When data is recorded on the recording medium, the magnetic film 2 is magnetized in advance in a predetermined direction, and a light beam intensity-modulated with a data signal is radiated thereon. A portion irradiated with the light beam is heated to near a Curie temperature, and its coercive force is decreased. Thus, an inverted magnetic domain is formed by an externally applied magnetic field. The inverted magnetic domain serves as a recording pit indicating data. The recorded data can be read out using the magneto-optical effect upon radiation of linearly polarized light beam.
In the opto-magnetic recording medium, in order to improve recording sensitivity, a magnetic film having a low Curie temperature must be used. On the other hand, in order to perform read access with a high S/N ratio, the higher magneto-optical effect is preferable. The level of the magneto-optical effect is represented by a Karr rotation angle indicating the degree of rotation of the radiated light beam polarization direction by a magnetic film. However, in general, if the rare-earth iron alloy has a larger Karr rotation angle, it tends to have a higher Curie temperature. Therefore, it is difficult to obtain a medium which can satisfy both the requirements of high recording sensitivity and a high reproduction S/N ratio.
In Japanese Patent Laid-Open Application No. 78652/1982 (U.S. counterpart application: U.S. Ser. No. 908,934 which is the continuation appln. of the CIP appln. Ser. No. 644,134 which is the CIP of U.S. Ser. No. 315,467), an optomagnetic recording medium having two magnetic layers is proposed, as shown in FIG. 2. Referring to FIG. 2, a second magnetic layer 4 of GdFe, GdFeCo, or the like, a first magnetic layer 5 of TbFe, DyFe, or the like and a protective layer 3 of a dielectric material are formed on a substrate 1. The second magnetic layer 4 has a lower coercive force and a higher Curie temperature than those of the first magnetic layer 5. These magnetic layers are exchange-coupled to each other. Recording is performed by heating these magnetic layers near the Curie temperature of the first magnetic layer by light beam radiation. Thus, an inverted magnetic domain is simultaneously formed in the two layers by an exchange-coupling force. In reproduction, a light beam is radiated onto the second magnetic layer 4 from the side of the substrate 1, thereby reading out data with a high S/N ratio. The double-layered magnetic layer structure is also described in "Magnetization Process of Exchange-Coupled Ferrimagnetic Double-Layered Films" Kobayashi et al., Japanese Journal of Applied Physics Vol. 20, No. 11, November, 1981, pp. 2089-2095 and "Thermomagnetic Writing on Exchange-Coupled Amorphous Rare-Earth Iron Double-Layer Films" Tsunashima et al., IEEE Transactions on Magnetics, Vol. MAG-17, No. 6, November 1981, pp. 2840-2842, in addition to the above-mentioned patent.
When an opto-magnetic recording medium is used in practical applications, an anti-corrosion property of the magnetic layer is important. However, in the medium with the double-layered structure, in particular, the first magnetic layer is easy to corrode, and long-term use cannot be ensured.